In industrial aluminium reduction cells or pots, where aluminium oxide is dissolved in a bath based on molten halides, one of the main concerns of the operator and also the main difficulty is to monitor the proportion of oxide ions (dissolved alumina) in the electrolyte. An excess of oxide which is soluble to about 7-8% results in the formation of sludge in the bottom of the crucible with highly prejudicial consequences in regard to progress of the electrolysis operation, while impoverishment of oxide ions results in a polarisation action, referred to as an "anode effect" which interferes with the operation of the cells. The anode effect is well known in aluminium smelters. It is related to current density or, in effect, applied voltage and causes the cell resistance to exhibit a sudden large increase above a critical current density
Direct and indirect methods and apparatus for determining the amount of dissolved oxide ions have been proposed. Indirect methods include those disclosed in U.S. Pat. No. 4,814,050 (Alcoa) and U.S. Pat. No. 5,025,219 (Pechiney). Direct methods are disclosed in U.S. Pat. No. 3,471,390 (Reynolds; inventors Kibby et al), U.S. Pat. No. 345,063 (Reynolds) and U.S. Pat. No. 4,935,107 (Pechiney). U.S. Pat. No. 3,471,390 discloses apparatus including a probe or test electrode having a cathodic graphite tip, a boron nitride spacer, and an anode, serving as a miniature reduction cell within which a rising voltage will elicit a rising current until such time as the "anode effect" appears, causing a rise in resistance and hence a drop of current. This effect depends primarily on the concentration of alumina in the electrolyte. Kibby's electrode is a plane surface on the end of a rod, having a graphite centre surrounded by an insulating ring. Kibby et al prefer to apply voltage changes in discrete steps over a predetermined period of time, and data is taken from the instrumentation by reading the dials of voltage and current meters. However, the extended duration of a single reading tends to result in significant electrode consumption and therefore the calibration drifts with time. The direct methods disclosed in the other patents referred to above also lack reproducibility and are therefore are not widely used.